The primary chemical method for making pulp from wood involves the digestion of lignin in the wood with sodium sulfide and sodium hydroxide. This is termed the sulfate or kraft process.
Wood pulp produced in the kraft process generally contains 5-8% by weight of residual, modified lignin which gives pulp a characteristic brown color. To obtain pulp of very high brightness and brightness stability, the lignin must be removed by certain oxidizing agents commonly referred to as bleaching chemicals. Many bleaching processes exist but almost all begin with the chlorination-extraction (C-E) stage. The spent liquor from the first alkali extraction stage of bleaching following chlorination, commonly referred to as E1 effluent, contains over 70% of the effluent color emanating from a kraft bleach plant (Alberti, B. N. and Klibanov, A. M. [1981] Biotech. and Bioeng. Symp. 11: 373-379). The effluent must be discharged due to its high content of corrosive chlorides. Polymeric lignin degradation products, the main contributors to color of bleach plant effluents, are resistant to the current bacteria-based effluent treatment process. Alternate treatment processes such as ultrafiltration, carbon adsorption, and massive lime precipitation are required for effective color removal, but are quite expensive. Economical color-removal systems do not presently exist and would be desirable for effluent treatment prior to its discharge to receiving waters.
Fungal decolorization systems have been studied. In USDA sponsored laboratory experiments (Kirk, T. K. [1983] in The Filamentous Fungi, Vol. 4, Fungal Technology, Smith, J. E., Berry, D. R., Kristiansen, B., eds., Edward Arnold Press, London), greater than 80% decolorization of bleaching effluent prepared by chlorination and alkali treatment of kraft-cooked synthetic lignins has been achieved in 24 hr using Phanerochaete chrysosporium cultures.
There are three problems in using fungal cultures to decolorize bleach plant effluents: (1) fungi require careful culture conditions (i.e., humidity, aeration, temperature and pH) not compatible with industrial processing environments; (2) fungi require long lag times and then only very slowly degrade lignin; and (3) fungi cannot grow on lignin. An additional food source must be added to support fungal growth.